round trip assembly materials when loading from step file

[shimwell]

When making assembly objects I am making use of the option to add materials like this

import cadquer as cq
result = cq.Workplane().sphere(5)
assembly = cq.Assembly()
assembly.add(result, material=cq.Material("mat1")) 

I would be keen to then save this assembly to a step file and load it up again later.

assembly.export("assembly_step1.step")

I am then loading it up and looking for the materials but the "mat1" is not found in the assembly anymore. I have tried two ways

from cadquery import importers
cadquery_object = cq.Assembly().importStep('assembly_step1.step')
cadquery_object = importers.importStep('assembly_step1.step')

for child in cadquery_object.children:
    print(child.material)

The child doesn't appear to have a material attribute.

I would be keen to be able to round trip the assembly so that I can save and load and get the same assembly back when loading